The conventional oscillators which equip mechanical timekeepers traditionally comprise a spring or balance-spring element making it possible to return a regulator or balance element to the neutral position. The energy dissipated by the oscillation is offset by the application of a drive torque provided by a loading spring, or a barrel spring. However, this drive torque exerted by the barrel spring varies over time according to the load (or degree of winding) of the latter part and, in most mechanical timekeepers, in particular when the barrel is coupled directly to the trains of the drive train, this variation has the effect of modifying the oscillation amplitude as well as, to a certain extent, the period of the oscillator. A modification of this type may translate, for certain embodiments, to a deviation from one to several tens of seconds per day.
In order to offset the effect of the variation of intensity of the drive torque, it was proposed to use a device called “fusee” (see the “Dictionnaire professionnel illustré de I'horlogerie” by G. A. Berner), which makes it possible to equalize the driving-power transmitted to the train by the barrel spring. However, a device of this type is difficult to miniaturize, and for this reason cannot actually be applied in mechanical watches.
Another corrective device was described in relation with FIG. 7 of European patent application EP 1 736 838 in the applicant's name. In this document, it is proposed to have the drive torque of the barrel spring act on a flexible organ, which controls the active length of an element which participates in the oscillation-constant of the mechanical oscillator. As in the case of the fusee, such a device is not easy to implement.
Also known from patent CH 279 954 is a mechanical oscillator comprising an oscillating system formed by a balance and its return spring and a frequency correction device. The corrector is based on controlling the active length of the return balance-spring by a mechanism controlled directly by the rotation of the winding pivot of the mainspring, which depends on the drive torque.
However, none of these corrective devices allow one to take variations in torque due to friction existing, for example, at the different parts, including the oscillator as well as the trains transmitting the drive torque to the latter, into account.
In quasi-permanent oscillation regime, i.e. when the intensity of the drive torque varies sufficiently slowly in relation to the oscillation period, one can allow that the period variation caused is equivalent to that which would be caused by a non-linear return torque according to the deflection. This type of isochronism defect can be corrected by a non-linearity which is the reverse of the return spring.